1. Field of the Invention
The present invention relates to an exhaust purification apparatus for an engine, and more specifically, to an exhaust purification apparatus in which a selective reduction-type NOx catalyst that selectively reduces NOx contained in exhaust gas by using ammonia as a reducing agent is interposed in an exhaust passage of an engine.
2. Description of the Related Art
A NOx catalyst that is interposed in the exhaust passage of an engine and selectively reduces NOx contained in exhaust gas has been known as a device for purifying engine exhaust. When this kind of a selective reduction-type NOx catalyst is utilized, urea-water is injected from an injection nozzle that is set upstream of the NOx catalyst in the exhaust passage. The injected urea-water is hydrolyzed by exhaust gas heat and the water vapor contained in exhaust gas, and ammonia (NH3) is produced. By using the ammonia (NH3) thus produced as a reducing agent, the NOx contained in exhaust gas is selectively reduced by the NOx catalyst.
In order to control the selective reduction-type NOx catalyst to deliver satisfactory performance of exhaust purification that is carried out by selectively reducing NOx, the selective reduction-type NOx catalyst has to be kept at a temperature equal to or higher than activation temperature (200 degrees centigrade or higher, for example). However, depending upon vehicle running conditions such as traffic jams in urban districts or environmental conditions such as cold regions in which outdoor air temperature is low, the temperature of the NOx catalyst may fall below the activation temperature. There has been the problem that, in such a low temperature range, the exhaust purification efficiency of the NOx catalyst with respect to NOx drastically decreases, and that a NOx emission amount into the atmosphere is increased.
In consideration of this problem, for example, Unexamined Japanese Patent Application Publication No. 2004-239109 (hereinafter, referred to as Document 1) proposes a measure for well maintaining the performance of exhaust purification carried out by selectively reducing NOx even in a situation where the temperature of the NOx catalyst is decreased.
According to the technology described in Document 1, a pre-oxidation catalyst is placed in an exhaust port of each cylinder of a diesel engine. In an exhaust passage connected in common to each of the exhaust port, there are disposed the urea-water injection nozzle, a pre-selective reduction-type NOx catalyst, a main selective reduction-type NOx catalyst, and a main oxidation catalyst, in the order from upstream to downstream. The pre-oxidation catalyst has a function of oxidizing a portion of NO contained in exhaust gas into NO2. This oxidative reaction of NO occurs even at low exhaust temperatures. The NO2 thus produced is supplied to the pre-selective reduction-type NOx catalyst and the main selective reduction-type NOx catalyst together with the ammonia produced from the urea-water. By using the NO2, these NOx catalysts offer an exhaust purifying operation that selectively reduces NOx.
Since the technology described in Document 1 uses the NO2 produced in the pre-oxidation catalyst, it realizes the exhaust purifying operation that selectively reduces NOx within a lower temperature range, as compared to conventional technologies. On the other hand, Document 1 is similar to conventional art in that the pre-selective reduction-type NOx catalyst and the main selective reduction-type NOx catalyst have low temperatures. As Document 1 says, the technology described is only capable of lowering the lower limit of a temperature rage, in which the exhaust purifying operation that selectively reduces NOx can be achieved, to around 180 degrees centigrade, and is not capable of dramatically improving such exhaust purification performance in the low temperature range.
In the technology described in Document 1, the pre-oxidation catalyst is required to be separately disposed in the exhaust port of each cylinder of the engine for creating NO2. For this reason, the technology described in Document 1 has the problem that the exhaust purification apparatus becomes greatly complicated in configuration and then that the production cost is inevitably increased.